This invention relates to cardiac pacers. More specifically, this invention relates to programmable cardiac pacers using isolation circuits to avoid cross-talk.
The heart serves as the pump which forces the blood through the blood vessels to all organs of the body. It is divided into four chambers, the two atria and the two ventricles. The atria serve as entryways to the ventricles and pump weakly to help move blood into the ventricles. The ventricles are the main pumps forcing blood to the rest of the body. For optimum cardiac output, the heart must contract in a coordinated manner with the atria contracting first and the ventricles soon afterwards.
The heart has an intrinsic pacemaker, located in the right atrium, which sets the heart rate and an intrinsic conduction system which maintains the required coordination among the chambers and among the individual muscle fibers of the heart. When the intrinsic pacemaker or the conduction system operates improperly, a man-made pacemaker may be implanted to take over the impaired functions.
A single chamber pacer may be used to stimulate either the right atrium or right ventricle. If the atrium is paced, the internal conduction system of the heart is depended on to carry the stimulus to the ventricle. If the conduction system between the atria and ventricles is not intact, the ventricle must be stimulated directly. An additional lead can then be placed in the atrium to sense normal atrial activity so that the pacer can stimulate the ventricle in synchrony with the atrial activity. If there is inadequate spontaneous atrial activity, the pacer can take over the function of the intrinsic pacemaker and stimulate the atrium followed by the ventricle after an appropriate delay.
Cardiac pacers may be classified as unipolar or bipolar, depending upon the configuration of the common electrode. A unipolar pacer uses the can or housing of the pacer as the common electrode and the tip of the lead as the other electrode. Since the myocardium is generally paced with a negative going pulse, the can serves as a positive ground. During sensing, the measured potential is between the tip of the lead and the can. A bipolar pacer has two electrodes on the lead, one at the tip and the other a short distance proximal. During pacing, the tip of the lead is the negative terminal and the proximal electrode is the positive electrode of the pulse generator. During sensing, the potential difference between the two electrodes on the lead is monitored. Thus, in bipolar sensing or pacing, the can is not part of the electrical circuit.
There are certain advantages to the bipolar configuration. During sensing, the system is less susceptable to environmental and muscle electrical noise pickup. During pacing, the bipolar configuration is less likely to cause unwanted skeletal muscle stimulation than the unipolar configuration. There are, however, also drawbacks to the bipolar configuration. The sensed signals are somewhat more variable than with unipolar sensing. Pacing is less energy efficient with bipolar stimulation and it is more difficult to detect the pacing artifact on a surface electrocardiogram.
It should be noted that a pacer may pace in one mode (unipolar or bipolar) and sense in the other mode.
Cardiac pacers having both atrial and ventricular channels are in common use. Usually each of the channels includes an associated lead which carries cardiac signals from the cardiac chamber to the channel sensing circuit. Additionally, each channel subsystem includes a stimulator which uses the lead for application of the stimulating pulse to the cardiac chamber, as necessary. However, in certain dual chamber pacing modes, the sense amplifier or stimulator may be absent or non-functional in either channel.
Cross-talk has posed significant problems in the operation of such pacers when the two channels are not effectively isolated. Specifically, the application of a stimulating pulse by the pulse generator by way of the two electrodes at the end of a chamber lead may inadvertantly stimulate the other chamber due to return current in the other chamber lead and the resulting polarization of the electrode. Depending on the magnitude of the polarization, the other chamber may be stimulated and the desired rhythm of the heart could be disrupted. In addition to such cross-stimulation, the polarization of an electrode due to other channel pacing may be sensed as activity in the chamber served by the polarized electrode. Because demand type pacers rely upon their sensing circuits to determine when a stimulating pulse should be applied, such cross-sensing should be avoided.
One disadvantage of various prior art techniques is that the operation of one channel will be dependent upon the operation of the other channel. For example, prior art switching circuits to isolate the channels may require that the sensing of the atrial cardiac signals stop when the ventricular stimulator is applying a stimulating pulse. This undesirable interdependence between the channels also commonly leads to complexity of design. More generally such complexity is a disadvantage of numerous prior art pacers including some lacking this interdependence.
Prior art systems using resistors alone or with other components to prevent cross-talk are disadvantageous in that high values of resistance introduce significant thermal noise into a circuit. However, high values of resistance may be necessary to provide the required isolation. Such high value resistors often diminish the signal magnitude of the sensed cardiac signals, effectively decreasing the ability of the system to sense cardiac signals. This and other factors constrain the selection of values for components such that such prior art systems may, of necessity, allow at least some cross-talk.
Although prior pacers have provided for some degree of programmability, the progammability of the pacers has been generally quite limited. For example, one prior art design requires that both channels (atrial and ventricular) be operated in the same mode with respect to the type of pacing (unipolar or bipolar) and the type of sensing. Other prior designs have limited one to programming the pacing, but not the sensing or alternately programming the sensing, but not the pacing.
One factor in limiting the programmability of the prior cardiac pacer designs has been the problem of cross-talk between the atrial subsystem and the ventricular subsystem. That is, use of a particular pacing mode on one channel may cause such significant cross-talk as to preclude use of a particular mode of sensing on the other channel.
A further disadvantage of numerous cardiac pacer designs is that cross-talk is prevented by turning off the sensing circuit of one subsystem (either atrial or ventricular) when the other subsystem is pacing. This results in a loss of information to the sensor which is turned off.